Most light emitting devices (LEDs) emit incoherent light. One performance measure of an LED is photometric efficiency, e.g. the conversion of input energy into visible light. Photometric efficiency is inversely proportional to the junction temperature of the LED. A major concern of the LED package designer is keeping the die cool to provide good overall performance.
For low power LEDs, e.g..ltoreq.200 mW, or die with small area, a large optical cavity size limits the ability to meet the desired reliability conditions. A smaller than optimum cavity size reduces the light extraction efficiency, e.g. the amount of generated light that exits the device. The prior art packages e.g. T1-3/4 and SnapLED, use cast epoxy as the hard encapsulant. The cast epoxy provides both optical and structural functionality. A prior art package design for an LED is shown in FIG. 1. The die is seated at the base of the optical cavity. A hard encapsulant, e.g. rigid unfilled epoxy, fills the optical cavity. Because the die, the optical cavity, and the encapsulant have different thermal coefficients, they expand and contract at different rates during operation. This places a high mechanical stress on the LED. In addition, the prior art packages lack thermal isolation between the electrical and the thermal paths because the electrical leads are the primary thermal paths. As a result, the packaged die are subject to thermal stresses from the temperature cycling, especially during assembly into end products.
These problems are exacerbated as the die increases in area or input power. Because a device having a larger junction area, e.g.&gt;0.25 mm.sup.2 requires a larger optical element than a small die, e.g.&lt;&lt;0.25 mm.sup.2, to provide comparable light extraction efficiency, a large optical cavity is necessary. The mechanical stress applied to the LED increases with the volume of the encapsulant. In addition, the stress increases as the packaged LED is exposed to temperature cycling and high moisture conditions. The accumulated mechanical stresses reduce the overall LED reliability.
Since prior art packages use their electrical leads as primary thermal paths, the high thermal resistance of these paths combined with the high thermal resistance of the external system creates high junction temperatures, when power dissipation increases, e.g..gtoreq.200 mW. High junction temperature contributes to accelerating the irreversible loss of photometric efficiency in the LED chip and also accelerates processes that contribute to the failure of mechanical integrity of the LED package.
None of the available LED packages provide reliable optically efficient operation for applications approaching LED average input power of 0.2 W, especially when operating under high (&gt;35%) duty factors or long pulse widths&gt;1 second.